We could add another level, but I think that we also need to think about ways of making the lower levels more challenging. Some ideas that we have are:
1) Face up to 5 monster groups instead of 3.
2) Lower the power of the controlled fireball. We were thinking that maybe it would hit the first group with full power, and then each subsequent group reduced somewhat. Also, perhaps, make some monsters that are totally immune to fireball.
3) Increase some of the monsters armor class, making them harder to hit. Currently the best armor class for a monster is -15 (which was an artifact of the original PC game), but we could make them much lower
4) Allow the monsters to sometimes get the first swing (although this could be pretty deadly).
We could potentially add additional magic items that could be used to counter some of these things.
What do you think? Do you have any other ideas?